A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In general, a lithographic apparatus includes one or more object tables arranged to hold a substrate or a patterning device. Such an object table can, in general, be displaced using a stage apparatus. Often, such a stage apparatus includes electromagnetic motors and/or actuators for displacing and positioning the object table. In order to obtain the required positioning accuracy, a combination of both electromagnetic motors (suitable for displacing the object table over comparatively large distances) and electromagnetic actuators (suitable for displacing the object table over comparatively small distances with a high accuracy) is often applied.
As an example of such a stage apparatus, a planar motor as e.g. disclosed in U.S. Pat. No. 6,531,793 (suitable for positioning an object table over comparatively large distances) can be combined with a plurality of electromagnetic actuators for accurate displacement of the object table.
In general, the electromagnetic motor that is arranged to provide a long travel to the object table includes an array of alternating polarized permanent magnets on a first part and a plurality of coils on a second part. During operation of the electromagnetic motor, an electromagnetic actuator that is operated or located in the vicinity of the electromagnetic motor may experience a disturbance caused by the electromagnetic motor. It will be appreciated by a person skilled in the art that an electromagnetic actuator that is displaced relative to an array of permanent magnets can be influenced by the magnetic field of the array. As such, the proper operation or expected response of the electromagnetic actuator may be compromised by the electromagnetic field of the array resulting in a reduced accuracy of the electromagnetic actuator. Similarly, the electromagnetic field generated by a current carrying coil of the electromagnetic motor may cause a disturbance or an improper operation of the electromagnetic actuator.
In order to avoid the influence of an electromagnetic motor on an electromagnetic actuator, one might consider providing a ferromagnetic shield between the motor and the actuator. Such a shield however may be quite heavy and therefore result in an inferior behavior of the stage apparatus. In addition, such a ferromagnetic shield may also be influenced by the electromagnetic field of the motor resulting in the generation of disturbances of the stage apparatus by the shield.